1. Field of the Invention
This invention relates to a method of recognizing an irradiation field in the case where a radiation image is recorded on a recording medium such as a stimulable phosphor sheet by limitation of the irradiation field of a radiation.
More particularly, this invention relates to a method of recognizing the irradiation field which can be effectively applied to both a case where only a single radiation image is recorded on one recording medium (This will be referred to as "single recording", hereinbelow.) and a case where the recording region on one recording medium is divided into a plurality of subdivisions and a plurality of radiation images are recorded on the respective subdivisions (This will be referred to as "subdivision image recording", hereinbelow.).
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, image processing is carried out on the electric image signal, and the radiation image of the object is reproduced as a visible image by use of the processed image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, the level of the radiation energy stored on the stimulable phosphor sheet is caused to fluctuate among radiation images by changes in the object, the image recording portion thereof, radiation dose, or the like.
However, in the aforesaid radiation image recording and reproducing system, characteristics of the stored image information of each radiation image, particularly the level of the radiation energy or the like of each radiation image stored on the stimulable phosphor sheet, may be ascertained in advance, and the light emitted by the stimulable phosphor sheet may be photoelectrically detected by use of read-out conditions such as a read-out gain and a scale factor adjusted to appropriate values in accordance with the characteristics of the stored image information of each radiation image. In this case, for each radiation image, it becomes possible to obtain a visible image free from adverse effects of the fluctuation in the level of the radiation energy stored on the stimulable phosphor sheet and suitable for viewing, particularly for diagnostic purposes, for example, a visible image wherein the necessary object image information is always expressed within the correct density range suitable for viewing, particularly for diagnostic purposes.
Also, in the aforesaid radiation image recording and reproducing system, image processing of the image signal detected photoelectrically is carried out by use of image processing conditions such as gradation processing conditions adjusted for each radiation image based on the image recording portion of the object such as the head, chest or abdomen, and/or the image recording method such as plain image recording or contrasted image recording so that a visible image suitable for viewing, particularly for diagnostic purposes, can be obtained. However, for example, in the case where detection of the image signal is carried out without using the read-out conditions adjusted in accordance with the characteristics of the stored image information of each radiation image, the image processing conditions should preferably be adjusted by considering the characteristics of the stored image information of each radiation image, which have been ascertained in advance, besides the image recording portion of the object and/or the image recording method. In this manner, it becomes possible to obtain a visible image suitable for viewing, particularly for diagnostic purposes, wherein the necessary object image information is expressed within the correct density range.
Ascertaining of the characteristics of the image information stored on the stimulable phosphor sheet prior to the image read-out and image processing may be carried out by use of the method as disclosed in U.S. Pat. No. 4,527,060. In the disclosed method, a read-out operation for ascertaining the image information of a radiation image stored on the stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out is carried out. The characteristics of the stored image information are ascertained based on the image information (preliminary read-out image signal) obtained by the preliminary read-out.
As mentioned above, the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read-out. Specifically, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out.
Another approach to the ascertaining of the characteristics of the image information stored on the stimulable phosphor sheet prior to image processing is to ascertain it based on the image information (image signal) detected by the final read-out. Though the characteristics of the stored image information ascertained in this manner cannot be used for adjusting the read-out conditions in the final read-out, they can be used for adjusting the image processing conditions. This method is effective in the radiation image recording and reproducing system wherein the preliminary read-out is not carried out.
Various methods have been proposed for ascertaining the characteristics of the stored image information based on the preliminary read-out image signal obtained by the preliminary read-out or the final read-out image signal obtained by the final read-out. As one of such methods, it has heretofore been known to utilize a histogram of the image signals (image signal levels). With this method, the characteristics of the stored image information may be ascertained based on, for example, the maximum signal value, the minimum signal value, or a signal value at which the frequency is the maximum in the histogram. Therefore, it becomes possible to reproduce a visible image having an improved image quality, particularly a high diagnostic efficiency and accuracy, by adjusting the final read-out conditions such as the read-out gain and the scale factor and/or the image processing conditions such as the gradation processing conditions and the frequency response processing conditions based on the maximum signal value, the minimum signal value, a signal value at which the frequency is the maximum, or the like in the histogram.
On the other hand, in the course of radiation image recording, it is often desired that portions of the object not related to diagnosis or the like be prevented from exposure to a radiation. Further, when the object portions not related to diagnosis or the like are exposed to a radiation, the radiation is scattered by such portions to the portion related to diagnosis or the like, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in many cases, the irradiation field is limited to an area smaller than the overall recording region on the stimulable phosphor sheet when a radiation image is recorded.
However, when the characteristics of the image information stored on the stimulable phosphor sheet are ascertained based on the histogram of the image signals, the problem as described below arises. As shown in FIG. 9, when an irradiation field 14 is limited to an area smaller than an image recording region 12 on a stimulable phosphor sheet 10 (In the case of the sheet 10 shown in FIG. 9, the image recording region 12 covers the whole area of the sheet 10.) and the preliminary read-out or the final read-out is carried out over an area markedly larger than the irradiation field 14, for example, over the overall image recording region 12 on the stimulable phosphor sheet 10, the characteristics of the image information actually stored within the irradiation field 14 are ascertained incorrectly. Specifically, in the aforesaid case, since the image signals at regions outside of the irradiation field 14 are also included in the histogram, the histogram does not accurately represent the actual image information stored within the irradiation field 14.
Therefore, in the case where radiation image recording is carried out by limiting the irradiation field, the characteristics of the stored image information are to be ascertained based on the preliminary read-out image signal or the final read-out image signal by the method as mentioned above, and the final read-out conditions and/or the image processing conditions are to be adjusted based on the ascertained characteristics, the irradiation field should be recognized and the characteristics of the stored image information should be accurately ascertained based only on the image signal within the irradiation field, thereby to eliminate adverse effects of the scattered radiation outside of the irradiation field.
Besides the case wherein the read-out conditions and/or the image processing conditions are to be adjusted for a radiation image stored on the stimulable phosphor sheet, recognition of the irradiation field is also necessary for various purposes in the case where a radiation image is recorded on a recording medium by limitation of the irradiation field.
The applicants have proposed various methods of recognizing the irradiation field 14 as disclosed in, for example, U.S. Pat. No. 4,851,678. However, though those methods can be suitably applied in the case of the single recording where only a single radiation image is recorded on one single recording medium and accordingly, only one irradiation field is on the recording medium, most of them are difficult to apply in the case of the subdivision image recording where a plurality of radiation images are recorded on a plurality of respective subdivisions of one recording medium with the irradiation field being limited for each recording and accordingly, a plurality of irradiation fields are on the recording medium.
For example, as a method of recognizing the irradiation field, there has been proposed an algorism utilizing Hough conversion. In the algorism, prospective edge points which are considered to be edge (contour) portions of the irradiation field on the recording medium are detected from image signals read out from the recording region of the recording medium, curves represented by formula EQU .SIGMA.=x.sub.o cos .theta.+y.sub.o sin .theta.
are obtained for the respective prospective points wherein x.sub.o and y.sub.o respectively represent the x coordinate (constant) and the y coordinate (constant) of a given prospective point when the positions of the prospective points are expressed with an x-y orthogonal coordinate system, straight lines defined by formula EQU .rho..sub.o =x cos .theta..sub.o +y sin .theta.o
in the orthogonal coordinate system (straight lines along the prospective points, e.g., straight lines 1.sub.1 to 1.sub.4 in FIG. 9) are obtained on the basis of the intersections of the curves thus obtained, and the regions surrounded by the straight lines or the regions surrounded by the straight lines and the outer edge of the recording region (in the case that the irradiation field 14 is positioned as shown in FIG. 10 with respect to the recording region 12 of the recording medium) are recognized as the irradiation field.
In this algorism, there can be a plurality of regions surrounded by the straight lines obtained by Hough conversion or by the straight lines and the outer edge of the recording region, and sometimes which region is the true irradiation field must be determined. For example, which region is the true irradiation field can be determined by detecting the center point of the recording region or the center of density (image signal level) gravity and recognizing the region including the center point or the center of density gravity as the irradiation center.
However, though the center point of the recording region or the center of density gravity is inherently positioned in the irradiation field in the case of the single recording, they are apt to be positioned outside the irradiation fields in the case of the subdivision image recording. Particularly, the center of density gravity can be positioned only in one of the irradiation fields in the case of the subdivision image recording.
Accordingly, though the irradiation field recognizing algorism can be suitably applied in the case of the single recording, a region outside the irradiation field can be mistaken for the irradiation field or only one of the irradiation fields can be recognized when the algorism is applied in the case of the subdivision image recording. That is, the algorism can be applied only when it is known in advance that the recording on a given recording medium is the single recording, and when the recording on a given recording medium is the subdivision image recording, or when it is not known which of the single recording and the subdivision image recording has been made on a given recording medium, the algorism is difficult to apply.